1. Field of the Invention
The present invention relates generally to batch type automatic icemakers adapted for installation in the freezer compartment of household refrigerators and, more particularly, to such an icemaker including a means to minimize supercooling of water in the mold ice-forming cavities.
2. Description of the Prior Art
A refrigerator automatic icemaker of the type shown and described in U.S. Pat. Nos. 3,163,017 Baker et al and 3,163,018-Shaw includes a mold having at least one ice-forming cavity. To begin the operation cycle, a means is included for filling the ice-forming cavity with a metered quantity of tap water. As the mold and water cools, an ice piece is formed. In order to initiate harvesting of the ice piece, a control means is included. The control means typically includes a temperature-responsive switch element (thermostat) in thermal contact with the mold. The thermostat is set to respond to a temperature well below 32.degree. F. It is assumed that when the mold temperature is below the set temperature, all the water has frozen into ice. A means in the form of an ice-ejecting pad normally positioned in the lower portion of the cavity is included to remove ice pieces from the cavity by raising them up out of the cavity clear of the mold to be swept into a storage bin by a sweep arm.
Although not generally appreciated, 32.degree. F more accurately represents the melting temperature of ice, rather than the initial freezing temperature of water, at least in the absence of a nucleating or "seeding" agent. In most cases, when a quantity of water is cooled for the purpose of freezing it, a temperature well below 32.degree. F is required to initiate freezing. A temperature as low as 25.degree. F is not at all unusual. This phenomenon of liquid water existing below 32.degree. F is known as supercooling and is the rule, rather than the exception. In order for ice crystals to form in water cooled to 32.degree. F or below, initial nucleation must occur. Initial nucleation is usually a random event, occuring at no particular temperature, and may be triggered, for example, by small foreign particles, mold irregularities, or mechanical movement. In the case of a foreign particle as a nucleating agent, the closer the crystal structure of the foreign particle is to the crystal structure of ice, the more effective it is and the less supercooling required before freezing occurs. In any event, the lower the temperature, the easier it is to initiate ice crystal formation. If the temperature is lowered sufficiently, eventually an initial ice nucleate forms spontaneously. It should be noted that, once an initial "seed" ice crystal is formed, the entire quantity of water can freeze with no further difficulty.
In an automatic icemaker of the above-described type, the thermostat which initiates ejection of the ice pieces from the mold cavities is typically set at 16.degree. F with a tolerance of .+-. 3.degree. F. As mentioned above, when the mold cools down to the set temperature, it is assumed that water in the mold cavities is completely frozen into ice. However, due to the supercooling effect, the mold and the liquid water in the ice-forming cavities can remain liquid even down to the temperature at which it is assumed that ice has been formed, and premature initiation of the ice-harvesting cycle occurs. This is particularly likely when a particular thermostat sample happens to respond to a temperature at the high end of the tolerance range, that is, 19.degree. F.
When this premature initiation occurs, the ejecting pads in the bottom of the ice-forming cavities rise up through liquid water, of course not removing any ice piece and having no real effect. When the next metered quantity of water enters the icemaker, since the cavities are already full, water simply overflows into the ice storage bin below, resulting in an unusable congealed mass of ice.
This problem of liquid water entering the ice-storage bin is particularly insidious because it occurs so infrequently, perhaps only once in every three or four hundred complete operating cycles in particular icemaker samples which are prone to it. As a result, the true cause is not apparent, especially since the sequence of events is rarely actually observed in an automatic icemaker. In an effort to "repair" the icemaker, parts such as switches and solenoid valves may be replaced, only to have another quantity of water mysteriously discharged into the ice-storage bin months later.
One way to make it statistically unlikely for supercooling to cause any problem is simply to employ a thermostat set to a very low temperature, for example 10.degree. F or lower, since it is unlikely that supercooling would continue to such a low temperature. The disadvantage of this approach lies in a decreased rate of ice production. It simply takes the mold and water or ice contained therein longer to reach such a low temperature, with no attendant advantage if ice has in fact formed.
Another approach might be the use of a particular chemical nucleating or "seeding" agent such as silver iodide or lead iodide. Such substances are known to initiate crystallization, causing liquid water to freeze into ice reliably at a relatively high temperature (still under 32.degree. F). While it might be possible to include such a nucleating substance within an icemaker mold, there are certain drawbacks to such an approach. For example, the substance chosen must have very low solubility in water so as not to be dissipated and, of course, must be nontoxic. Furthermore, no such material is as effective as ice itself in nucleating water close to 32.degree. F. This follows from the fact that these substances can only approach the structure of ice, but cannot be identical.